[@Ryougu] They actually do work with other guilds. Having created elementals with Izzet. They also have asked Selesnya for help in the past. They are tight knit with Azorius law mages who they police with.

However guilds like Rakdos and Dimir are seen as vermin to be destroyed. As they only exist to cause chaos.

Also if your guildless. (Like most of Ravnica.) The Boros are a symbol of protection.